Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 140746 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 346 AM EDT Sat Oct 14 2017 .SHORT TERM...
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(Today through Sunday) Issued at 346 AM EDT SAT OCT 14 2017 Early this morning, an upper level ridge was centered across the Southeast Conus while a shortwave trough was moving through the Northern Rockies and another moving through the Central Rockies. Another ridge was in place near the California coast. At the surface, high pressure was centered off the New England coast. Fog developed during the evening and overnight in the valleys, generally the deeper valleys along the larger creeks and area rivers. At observation sites, dense fog has not generally been reported, although dense fog has been reported at KSYM over the past few hours. Meanwhile, low level moisture has worked its way around the Southern Appalachians and is moving north across parts of East TN and the Cumberland Plateau. This is evident on GOES 16 nighttime cloud microphysics and 10.3-3.9 fog channel difference likely as stratus and stratocu. Recent ASOS and AWOS observations in eastern and middle TN have cloud bases between generally 3000 feet AGL or lower. The surface high will continue to depart to the north and east today with return flow of low level moisture into the region as an area of low pressure organizes over the Plains. The low is expected to track into the Great Lakes this evening and tonight reaching the St Lawrence Valley on Sunday. Meanwhile, a trough axis and the trailing cold front will begin to approach the region. The cold front should cross the area from midday on Sunday into Sunday evening. For today, the NAM and GFS bring the increase in moisture north in the 925 to 850 mb levels. This should lead to a further expansion of the stratocu north into the area this morning with this probably lifting into a rather extensive cumulus deck this afternoon especially across the south and southwest part of the area. With south to southwest flow in place, it will be another day with above normal temperatures. The models generally have a minimum in the low level moisture moving across the area this evening into the overnight hours at the same time the pressure gradient begins to increase across the area. This should set the stage for some southeastern valley locations decoupling for at least part of the night, possibly the entire night, with some fog forming in those locations. This nocturnal inversion would support a small to moderate ridge/valley split in places. The front and as associated mid level trough will approach on Sunday morning with the front moving across the area from about midday into the early evening. Showers should accompany the boundary and limited instability could support a couple of isolated thunderstorms. The models seem to indicate the boundary may interact with more surface heating in the central and eastern part of the area and thus pops were highest in those areas with likely across the remainder of the area. Max temperatures should be reached around midday to early afternoon and then the combination of wetbulbing and eventually cold air advection leading to falling temperatures from about mid afternoon on. The pressure gradient and winds aloft should also support sustained winds up to 15 mph ahead of the arrival of the showers with some gusts into the 20 to 30 mph range. .LONG TERM...(Sunday night through Friday) Issued at 255 AM EDT SAT OCT 14 2017 Cold front will exit off to the south early Sunday night, with any lingering precipitation quickly exiting by midnight. Much cooler weather will filter into the coal fields through the overnight hours with some of the high terrain in southeast Kentucky reaching into the upper 30s by Monday morning. All other areas should see widespread mid 40s. Despite sunshine returning on Monday, highs will be much cooler with readings only around 60. Dewpoints may mix out into the mid to upper 30s by Monday afternoon. This will set the stage for a combination of fog and frost for Monday night into early Tuesday morning. At this time, it looks like fog should be the primary issue near bodies of water, while some patchy frost may occur in more sheltered valleys. We should see better mixing and drying occurring on Tuesday with perhaps the coldest night of the week for the valleys coming Tuesday night. With the better mixing and drying, frost will be more of a concern for Tuesday night. Still not looking like a widespread frost at this point, but certainly going to be some frost in places (again, mainly just in the eastern valleys). A strong ridge will slowly migrate east across the region through the remainder of the week. This will allow for a slow warming trend through the rest of the week. Will continue to go slightly on the high side of guidance as the airmass will be very dry (not all that uncommon for this time of year). Also went lower on dewpoints each afternoon and may still not be low enough, but will certainly continue to trend the afternoons drier for potential fire weather concerns. Given the airmass, RH`s under 25 percent would seem very plausible by mid to late next week. At this point, no real signs of another chance of rain through the beginning of next weekend as the35 ridge remains anchored over the region.
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&& .AVIATION...(For the 06Z TAFS through 06Z Saturday night) ISSUED AT 205 AM EDT SAT OCT 14 2017 Fog has formed in the river valleys per observations and GOES 16 nighttime cloud microphysics RGB. Model forecast soundings indicate that fog should continue to be mainly shallow and confined to the deeper valleys. However, a south to southwest component of the low level flow has brought some fog into KSYM. The dewpoint depression is near 0 at KSME attm so some MVFR fog was included there later tonight. Otherwise, the fog is not expected to affect the TAF sites. Non TAF site valley locations will experience some vis down to the IFR range through 13Z and it could approach mins. Models continue to suggest a low cloud deck of stratus or stratocu developing or moving into the area through 15Z. Satellite trends in the TN Valley seem to generally support this, but confidence is low as to when and at what level these would develop. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...JP LONG TERM...KAS AVIATION...JP

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